[SFML C++ 2D] spawn falling rocks at random times

Hi, I have a rock object. Right now all it does is fall from the sky. I made 10 of them and put it in a std::vector<rocks*> rock_v. The rocks have random x position but start at the same height.
I generate the rocks like this:

for(int i = 0; i < 10; i++)
{
rock_v->init();
}

The problem with this is all the rocks will spawn at the same time (millisecond apart from each other). How can I make it so they fall at a certain interval -- say 0.5 sec.